The catalytic cracking device is the main device for producing gasoline, and a majority of motor gasoline in the world comes from the catalytic cracking device, and a riser reactor is employed for conventional catalytic cracking.
The biggest shortcoming of the existing riser reactor lies in that the riser is too long. The catalyst activity at the outlet of the riser is only about one-third of the initial activity for the catalyst. Therefore, the activity and selectivity of catalyst have been dramatically reduced in latter half part of the riser reactor so that catalysis degrades and the thermal cracking reactions and other detrimental secondary reactions increase. It not only limits the increase of the single-pass conversion of the raw materials, but also simultaneously causes the olefin content of cracking gasoline to be up to 45% or more, thus far from meeting the requirements for new gasoline standard. With the reduction in catalyst activity, the selectivity of catalytic reaction is inevitably reduced, and side reactions increase naturally.
In order to improve the single-pass conversion in catalytic process, a key problem is to enhance the catalyst activity in latter half part of the existing riser reactor. CHINESE patent application No. 99213769.1 discloses a two-stage series-connected apparatus for catalytic cracking which comprises two identically structural catalytic cracking apparatuses vertically overlapped one upon another. By vertically overlapping the reaction and regeneration apparatuses one upon another, this technique intensifies the catalytic cracking process in the conventional riser by shortening reaction time, thereby improving the effective activity and selectivity of the catalyst. However, the technique disclosed in this patent application is merely limited to theory, and lacks the operable implementation method. The implementation of this technique corresponds to constructing two vertically overlapping reaction-regeneration apparatuses for catalytic cracking with higher investment, thus it is less likely to be implemented.
CHINESE patent application No. 00122845.5 discloses a two-stage catalytic cracking process for hydrocarbon oil as follows. Hydrocarbon oil firstly contacts and reacts with a cracking catalyst in a first reactor, and thus generated oil-gas is conveyed to a second reactor to contact and react with a catalyst containing high silica zeolite of five-membered ring, and thus generated oil-gas is then conveyed to a fractionating tower for separation. The catalysts in the two reactors are different in composition and property in this method. Although the product selectivity in the second reactor is enhanced by allowing the reacted oil-gas in the first reactor to be in contact with the fresh catalyst in the second reactor, two kinds of catalysts and two parallel-arranged reaction-regeneration systems make the investment cost higher.
CHINESE patent application No. 00134054.9 discloses a new catalytic cracking technique using a two-stage riser in which a riser is divided into an upper stage and a lower stage. Catalyst in a first stage comes from a regenerator, and after the reaction in the first segment ends, the catalyst and oil-gas are separated through an intermediate separator arranged at the end of the first stage with only the oil-gas continuing to enter the second reaction stage for reaction; the catalyst in the second reaction stage is a regenerated catalyst from the regenerator which is subjected to a heat exchange via an external heat exchanger. This technique is to allow high active and cooled low-temperature regenerated catalyst to continue to contact and react with the oil-gas in the second reaction stage (i.e., the latter half part of the riser), whereby the catalyst activity in the second stage and single-pass conversion are improved. However, the catalyst separated from the first stage is necessarily subjected to a steam stripping before entering the regenerator in this technique, and meanwhile the regenerated catalyst must be conveyed upwards by a conveying medium to be able to enter the second stage, and both stripping steam and the conveying medium will enter the riser in the second stage, which will affect the reactions in the second stage inevitably; if the amount of stripping steam is restricted, it will then affect stripping effect, and further affect the regeneration procedures; in addition, a height difference from the bottom of the external heat exchanger to the inlet of the second stage is up to tens of meters and there needs a large number of conveying medium, so a large amount of power consumption is required; and the investment will be largely increased, because two settlers and two stripping sections are required in this technique.